The present invention relates to an optimum start system and, more particularly, to such a system in which the amount of lead time before occupancy during which the temperature control system is energized to bring the space temperature to within the temperature comfort range established for the building is determined as a function of space temperature, outdoor air temperature and an accuracy factor which is based upon the accuracy of at least one prior lead time.
Most control systems which control the condition of air within commercial buildings today have various energy management functions which the control systems can perform. For example, the setpoint temperature within the building can be set-up during summer months and setback during winter months when the building is not occupied in order to reduce the cost of running the air condition control system during times of non-occupancy. During non-occupancy, the setpoint temperature established for a space temperature within the building is established at a level which will cause the temperature control system to operate at a more efficient energy level. More precisely, during winter months, the setpoint temperature during periods of non- occupancy is lowered so that the heating plant will run less and consequently require less energy. On the other hand, during summer months, the setpoint temperature during periods of non-occupancy is raised so that the chiller plant likewise will run less and require less energy.
Setpoint temperature can be defined as not just a single temperature but rather a range of temperatures so that if the space temperature is located within the range, no temperature treatment of the air is required and thus energy will be conserved. For example, the comfort range for the space temperature within a building may be established between 68.degree. F. and 78.degree. F. For purposes of this invention, the lowest temperature of the comfort range will be defined as the low comfort limit and the highest temperature in this range will be defined as the high comfort limit.
Typical temperature control systems with set-up and setback functions can perform an optimum start function. The set-up temperature may be either a specific temperature to which space temperature is allowed to drift during summer unoccupied periods or it may be no defined temperature in those systems in which space temperature is allowed to uncontrollably drift during summer unoccupied periods. According to the optimum start function, the temperature control system reestablishes the comfort range as the setpoint temperature an amount of lead time prior to the start of occupancy so that the space temperature can be brought back from the unoccupied temperature to within the comfort range by the time that occupancy begins. Prior optimum start systems of this nature have relied upon outdoor air temperature to determine this amount of lead time. For example, during the heating months, the colder the outdoor air temperature, the more lead time is necessary during the start-up period in the morning to make sure that space temperature is brought up to within the comfort range by the beginning of occupancy. The use of outdoor air temperature by itself to determine lead time is not ideal because each building, because of its construction and because of external factors other than outdoor temperature, such as wind and solar radiation, does not have the same heat transfer characteristics for the same outdoor air temperature.
Accordingly, optimum start functions have been developed to use space temperature drift rate in order to determine the lead time. In optimum start applications, for example, it has been suggested to turn on the temperature control system sometime during non-occupancy either for a fixed period of time and to measure the temperature change during that period of time or for a variable period of time necessary to change the temperature by a fixed amount. The temperature change is then divided by the period of time in order to determine drift rate (i.e. the rate of temperature change). This rate of temperature change is thus dependent upon building construction and most if not all external conditions. The temperature control equipment is normally turned on during the period of non-occupancy an amount of time prior to occupancy time which is greater than the greatest possible lead time. The difference between the desired temperature at occupancy and the current space temperature is then divided by the drift rate in order to determine the amount of time necessary to bring space temperature up to the desired occupancy temperature. The lead time is substracted from the occupancy time in order to determine the start time at which the setpoint of the temperature control system must be changed in order to bring space temperature up to within the comfort range by the beginning of occupancy.
Such a system requires that the temperature control system be cycled during the nighttime hours either routinely to maintain the setback or set-up temperature or specially because conditions are not such that cycling is required to meet the setback or set-up temperature. If it is undesired to turn the temperature control system on during the nighttime hours solely for the purpose of measuring drift rate, the drift rate can be determined during the last cycle before the temperature control system is in either setback or set-up. However, conditions can change drastically between the beginning of non-occupancy and the start of occpancy such that the use of drift rate resulting from the cycle just prior to the beginning of non-occupancy in determining lead time for the following occupancy period will result in error.
Also, lead time calculations can be erroneous, i.e. the temperature control system brought space temperature to within the comfort range either too early or too late. Accordingly, it is desirable to provide in the optimum start system the ability to learn from past mistakes and correct itself in future lead time determinations. Copending application Ser. No. 448,148 filed on Dec. 9, 1982 discloses one type of an adaptive optimum start system.